We've never participated in a gangbang, so can't offer any first hand experience there. I can say that like you, I would have felt uncomfortable in that situation too, even if I felt sure that being treated like that was something that I knew she wanted as part of the experience. It's just too far outside of "me" if that makes any sense. Now, if Mrs. cplnuswing wanted that, then I think I could do it just fine, but that's because I know her really well and so wouldn't have any of those qualms regarding: Is this what she wants, or is this what he wants? Is she really enjoying this, or is she just acting that way? Where's the line?
This one is maybe a gray area, but I've always thought that the further away you get from the middle of the road on sex, the more incumbent it is on you to make sure that everyone involved is cool with everything and that get done up front, not during or after. Problem there is what is middle of the road for one is plain vanilla or totally over the top for others, so it's kind of a hard call to make.
You also ask is this common in swinging, and to that I do have personal experience, and we've never encountered feeling like we were in a situation like that. So, I would take this as sort of an outlier, and use it as a learning experience to try to feel out a scene a little more before getting involved. Swinging encounters that don't go as well as hoped almost always have an element of lacking or missed communication to them, so don't feel bad, it's not uncommon and it happens to nearly everyone at one time or another.